Plastocyanin redox kinetics in spinach chloroplasts: evidence for disequilibrium in the high potential chain |
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Authors: | Helmut Kirchhoff Mark Aurel Schöttler Julia Maurer Engelbert Weis |
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Institution: | Institut für Botanik, Schlossgarten 3, D-48149 Münster, Germany |
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Abstract: | Reduction kinetics of cytochrome f, plastocyanin (PC) and P700 (‘high-potential chain’) in thylakoids from spinach were followed after pre-oxidation by a saturating light pulse. We describe a novel approach to follow PC redox kinetics from deconvolution of 810-860 nm absorption changes. The equilibration between the redox-components was analyzed by plotting the redox state of cytochrome f and PC against that of P700. In thylakoids with (1) diminished electron transport rate (adjusted with a cytochrome bf inhibitor) or (2) de-stacked grana, cytochrome f and PC relaxed close to their thermodynamic equilibriums with P700. In stacked thylakoids with non-inhibited electron transport, the equilibration plots were complex and non-hyperbolic, suggesting that during fast electron flux, the ‘high-potential chain’ does not homogeneously equilibrate throughout the membrane. Apparent equilibrium constants <5 were calculated, which are below the thermodynamic equilibrium known for the ‘high potential chain’. The disequilibrium found in stacked thylakoids with high electron fluxes is explained by restricted long-range PC diffusion. We develop a model assuming that about 30% of Photosystem I mainly located in grana end-membranes and margins rapidly equilibrate with cytochrome f via short-distance transluminal PC diffusion, while long-range lateral PC migration between grana cores and distant stroma lamellae is restricted. Implications for the electron flux control are discussed. |
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Keywords: | chl chlorophyll cyt cytochrome DNP-INT 2 4-dinitrophenylether of iodonitrothymol Δ? differential absorption coefficient Em redox midpoint potential ET electron transport FR far red P700 reaction centre chlorophyll of Photosystem I PAM pulse-amplitude-modulation PC plastocyanin PQ plastoquinone PS photosystem |
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